1. Field of the Invention
The invention relates to a digital radio modulator and in particular a phase modulator.
2. Description of the Prior Art
Direct digital modulation techniques have the advantage of eliminating a number of analogue components which are inherently less accurate than digital components and more susceptible to changes in characteristics because of temperature changes and ageing.
Phase modulation can be effected by synthesising separate in-phase and quadrature (sine and cosine) components of (a) intermediate frequency (IF) carrier signals and (b) modulation signals; mixing respective carrier and modulation signals; and summing the mixer outputs. This is an established analogue technique and proposals have been made to use a precisely analogous digital technique--see for example EP-A-0377180.
Such a system, however, requires high speed high capacity digital storage and processing with consequently many integrated circuit chips. Chip count can be reduced by very large scale integrated circuit (VLSI) techniques. An example of progress in this direction is described in U.S. Pat. No. 5,008,900 where a finite impulse response (FIR) chip is used in conjunction with a digital intermediate frequency (DIF) chip to perform digital IF synthesis and modulation. Again, however, the in-phase and quadrature components for the IF and modulating signals are derived and applied to a modulator in a manner analogous to the analogue process. Thus, the requirement for digital memory capacity is hardly reduced and the VLSI chips are very expensive. The present invention provides an improvement.